1. Field of the Invention
This invention relates to a solid state image sensor comprising a plurality of photoelectric conversion elements formed in a semiconductor substrate, and more particularly to a drain region for preventing a blooming phenomenon.
2. Description of the Prior Art
In general, in a solid state image sensor, signal carriers are stored in a plurality of photoelectric conversion elements on a semiconductor substrate according to optical information, and are read out by means of a charge coupled device (C.C.D.). In the case where the quantity of light incident on the photoelectric conversion element in a predetermined period of time is considerably great, carriers are produced in the phototlectric conversion element in excess of the maximum storage capacity, and accordingly the excess carriers are caused to flow into the adjacent photoelectric conversion element over a channel stop region which isolates the photoelectric conversion elements from one another. This is a so-called blooming phenomenon. There has been a strong demand for preventing the blooming phenomenon.
Heretofore, a method is employed to prevent the blooming phenomenon, in which a blooming phenomenon preventing drain is formed in contact with the channel stop region in the vicinity of the surface of the substrate, and a bias voltage is applied to the region, so that the excessive carriers are allowed to flow to the drain region. It is true that the blooming phenomenon can be prevented by this method. However, this method is disadvantageous in that the effective area of the photoelectric conversion element is reduced as much as the blooming phenomenon preventing drain is formed, and the effective sensitivity of the solid state image sensor is lowered. In addition, the method is disadvantageous in increasing the density of integrated circuits forming the solid state image sensor.